Edit: This has been solved. It is indeed once a week on 34 and 35, with an apparent showing factor of needing at least one Skolas completion.
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I'm not sure but it seems like quite a common drop on lv35 PoE. I gotten one Elder Cipher on my first run last week. On my second and third Skolas run three other people got the Elder Cipher. This week, I've gotten two Elder Ciphers out of three Skolas runs. During one of them the entire Fireteam got the Elder Cipher.